An innovative beam design promises strength and long service life for bridges.
Conventional steel-reinforced concrete bridge girders have a useful life of about 75 years, depending on traffic loads and weather. Seemingly impervious to the elements, “concrete itself is a giant sponge,” explains Mike Zicko, an engineer with HCB Company. Water, along with impurities from the roadway or deicing chemicals, is pulled in by the concrete and failure of bridges, whether it’s the deck, girders or other components is caused in most cases “by rusting of whatever metal is in the bridge.”
Rust causes the steel to expand and crack the concrete – protect the steel from moisture and the life of the structure is prolonged.
The second reason for structural failure of bridges is fatigue. “When something goes over a bridge, it takes away some of its life,” says Zicko. After a lifetime of bending under the weight of traffic, steel can wear out.
A new beam that uses Fiber Reinforced Polymer, a material that Zicko says is “virtually impervious to moisture” and stronger than steel may help bridges last up to 100 years. FRP is seeing increased use in the transportation and other industries partly because it does not rust. UDOT has recently used FRP to reinforce the deck of the Beaver Creek Bridge and FRP bars are being used to extend the life of concrete pavement on I-15.
Called a Hybrid Composite Beam, the design uses an FRP box to protect concrete used in the beam from moisture. What’s inside the box is innovative as well; a concrete arch gives the beam compressive strength.
More than just a covering, the box “provides shear strength and encapsulates the tension and compression elements,” according to the HCB Company website. The arch structure inside the beam is surrounded with low density foam core. A prestressing strand provides additional strength and steel shear connectors provide stiffness. Along with being very strong and durable, the beams are also light and easy to lift and place.
The beam was designed by structural engineer John Hillman, President and CEO of HCB Company. UDOT will use the beam on a bridge near Beaver, Utah. A grant from Highways for Life will provide funding to use HCBs on the project. HFL encourages state DOTs to use innovation to build a longer lasting transportation system.